Chemical zoning in igneous minerals is a potential record of the time, processes, and thermal evolution during the lifetime of a given magma reservoir. Abundances of volatiles (OH, Cl and F) in apatite from terrestrial and extraterrestrial plutonic and volcanic rocks have been used to study volatile behavior in magmas, however, volatile diffusivities in apatite are poorly constrained. Here we report new experimental results on Cl, F and OH diffusivities in apatite and apply them to estimate magma ascent times and rates. The experiments were carried out on oriented natural Durango fluorapatite crystals at 800–1100 °C, 1-atm, and oxygen fugacity at the wüstite-magnetite buffer. Experimental charges and chemical profiles were investigated with a variety of methods, including scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, secondary ion mass spectrometry, and nuclear reaction analysis.

We find that the concentration profiles of Cl show evidence of uphill diffusion that is likely related to the co-existence of three monovalent anions, i.e., OH⁻, Cl⁻, F⁻, at the same site of the apatite structure. Chemical gradients of OH, Cl and F were reproduced using a multicomponent diffusion model to extract the tracer diffusion coefficient ($D_i^{⁎}$) of each component (i). The calculated values of $D_i^{⁎}$ parallel to the c-axis show a general relation of $D_F^{⁎}>D_{Cl}^{⁎}>D_{OH}^{⁎}$, and define the following Arrhenius relations (parallel to the c-axis, at 1 bar) as:$D_{Cl}^{⁎}=7\left(\begin{array}{c}\hfill +12\hfill \\ \hfill -4\hfill \end{array}\right)\times {10}^{-5}\times \left[\mathit{exp}\left(\frac{-294(\pm 12)\text{ kJ}\mathbf{\cdot }{\text{mol}}^{-1}}{RT}\right)\right]{\text{ m}}^2\mathbf{\cdot }{\text{s}}^{-1}$$D_F^{⁎}=5\left(\begin{array}{c}\hfill +49\hfill \\ \hfill -5\hfill \end{array}\right)\times {10}^{-4}\times \left[\mathit{exp}\left(\frac{-308(\pm 76)\text{ kJ}\mathbf{\cdot }{\text{mol}}^{-1}}{RT}\right)\right]{\text{ m}}^2\mathbf{\cdot }{\text{s}}^{-1}$$D_{OH}^{⁎}=4\left(\begin{array}{c}\hfill +11\hfill \\ \hfill -3\hfill \end{array}\right)\times {10}^{-2}\times \left[\mathit{exp}\left(\frac{-401(\pm 39)\text{ kJ}\mathbf{\cdot }{\text{mol}}^{-1}}{RT}\right)\right]{\text{ m}}^2\mathbf{\cdot }{\text{s}}^{-1}$

The activation energy for Cl diffusion that we determined (294 $\text{kJ}\mathbf{\cdot }{\text{mol}}^{-1}$) is within the range of that reported by Brenan (1994), but the pre-exponential factor is smaller and thus we obtain in general slower diffusivities than Brenan (1994). $D_{Cl}^{⁎}$ and $D_{OH}^{⁎}$ parallel to the a-axis are 1 to 2 orders of magnitude slower than those parallel to the c-axis, indicating anisotropic diffusion of Cl and OH. Preliminary results on S diffusivity (parallel to the c-axis) at 800–900 °C show values between those of Cl and OH. The diffusion coefficients and model proposed in this study can be used to estimate the timescales of volatile re-equilibration in apatite in a variety of contexts from plutonic rocks and layered intrusions, to volcanic rocks and meteorites. We show that, for example, magma ascent rates can be determined by modelling Cl zoning in volcanic apatite. These applications provide new opportunities for understanding the influence of magma ascent rates on the eruption styles of volcanoes, thus having potential contributions to improving volcano forecasting and hazard assessments.

火成岩矿物中的化学分区可能是给定岩浆储层寿命期间时间，过程和热演化的潜在记录。陆生和外生的深部火山岩和火山岩中磷灰石中大量的挥发物（OH，Cl和F）已被用于研究岩浆中的挥发物行为，但是，磷灰石中的挥发物扩散受到的限制很有限。在这里，我们报告了有关磷灰石中Cl，F和OH扩散率的新实验结果，并将其应用于估算岩浆上升的时间和速率。实验是在800-1100°C，1-atm且在白铁矿-磁铁矿缓冲液中的氧逸度下对定向的天然Durango氟磷灰石晶体进行的。用各种方法研究了实验电荷和化学特征，包括扫描电子显微镜，透射电子显微镜，

我们发现，氯节目上坡扩散的证据的浓度分布是很可能与三一个价阴离子共存，即，OH ^- ，氯^-，F ^- ，在磷灰石结构的同一部位。使用多组分扩散模型重现OH，Cl和F的化学梯度，以提取示踪剂扩散系数（$d_{\mathrm{一世}}^{⁎}$）的每个组成部分（i）。的计算值$d_{\mathrm{一世}}^{⁎}$平行于c轴显示$d_F^{⁎}>d_{C升}^{⁎}>d_{ØH}^{⁎}$，并将以下Arrhenius关系（与c轴平行，在1 bar处）定义为：$d_{C升}^{⁎}=7（\begin{array}{c}\hfill +12\hfill \\ \hfill -4\hfill \end{array}）\times {10}^{-5}\times \left[\mathit{经验值}（\frac{-294（\pm 12）\text{ 千焦}\mathbf{\cdot }{\text{摩尔}}^{-\mathrm{1个}}}{\mathrm{[R}Ť}）\right]{\text{ 米}}^2\mathbf{\cdot }{\text{s}}^{-\mathrm{1个}}$$d_F^{⁎}=5（\begin{array}{c}\hfill +49\hfill \\ \hfill -5\hfill \end{array}）\times {10}^{-4}\times \left[\mathit{经验值}（\frac{-308（\pm 76）\text{ 千焦}\mathbf{\cdot }{\text{摩尔}}^{-\mathrm{1个}}}{\mathrm{[R}Ť}）\right]{\text{ 米}}^2\mathbf{\cdot }{\text{s}}^{-\mathrm{1个}}$$d_{ØH}^{⁎}=4（\begin{array}{c}\hfill +11\hfill \\ \hfill -3\hfill \end{array}）\times {10}^{-2}\times \left[\mathit{经验值}（\frac{-401（\pm 39）\text{ 千焦}\mathbf{\cdot }{\text{摩尔}}^{-\mathrm{1个}}}{\mathrm{[R}Ť}）\right]{\text{ 米}}^2\mathbf{\cdot }{\text{s}}^{-\mathrm{1个}}$

我们确定的Cl扩散的活化能（294 $\text{千焦}\mathbf{\cdot }{\text{摩尔}}^{-\mathrm{1个}}$）在Brenan（1994）所报道的范围内，但是前指数因子较小，因此我们得到的扩散率通常比Brenan（1994）更慢。 $d_{C升}^{⁎}$ 和 $d_{ØH}^{⁎}$平行于a轴的速度比平行于c轴的速度慢1至2个数量级，表明Cl和OH的各向异性扩散。S扩散率的初步结果（平行于C轴）在800–900°C下显示的数值介于Cl和OH之间。这项研究中提出的扩散系数和模型可用于估算磷灰石中挥发物再平衡的时间尺度，从矿物岩和层状侵入体到火山岩和陨石，磷灰石在各种情况下均能重新平衡。我们表明，例如，可以通过对火山磷灰石的Cl分区进行建模来确定岩浆上升速率。这些应用为理解岩浆上升速率对火山喷发方式的影响提供了新的机会，因此对改进火山的预报和灾害评估具有潜在的贡献。